Demigod

The Demigods are self-proclaimed successors to the "mortals" beneath them. In truth, they are mortal, but they have undergone a series of enhancements so extensive that they cease to be recognizable as still possessing mortal qualities. All of these come together to render the Demigods immortal. This is accomplished via the implantation of a new, synthetic organ called the "Vivimus" that acts as the housing unit for billions of nanites. These nanites have the sole purpose of repairing the telomeres - a compound structure at the end of a chromosome - thus causing cells to never undergo genetic deterioration upon their split, causing the Demigod in question to never age.
The Vivimus was created by Ma'kal Ji'ne, Ravedûn Mir, and Carlsbad Han largely on accident. Their original goal was to cure diseases resulting from apoptosis. The theory was that halting the cells' deterioration and death would solve the issue that led to the diseases. In the process, they created immortal cells, and, eventually, immortal people. They, naturally, used it on themselves, making them the first Demigods.